Among many uses for internal combustion engines, such engines are often employed for powering various vehicles, either as a primary power source, or as part of a hybrid powertrain. When an internal combustion engine is used in a hybrid powertrain, such an engine is combined with one or more electric motors to power the vehicle.
In order to maximize fuel efficiency of a hybrid vehicle, it is often desirable to shut the engine off and decouple it from the driven wheels when the vehicle is subject to deceleration. If, however, the powertrain is configured such that the engine may not be completely decoupled from the driven wheels, it is at least desirable to shut off the engine's supply of fuel, while permitting the engine to be motored or driven by the vehicle's inertia. When such a permanently coupled engine's fuel supply is shut off during vehicle deceleration, the engine continues to operate as an air pump which incurs spin and pumping losses and also increases engine braking. Generally, such losses reduce gains in fuel efficiency that may otherwise result from the engine being shut off. Additionally, such losses are likely to reduce the energy available to be recaptured by the powertrain's electric motor for recharging vehicle batteries.